Lumber which has recently been cut and machined contains a relatively large percentage of water and is referred to as green lumber. Prior to being used in construction or other applications which demand good grades of lumber, the green lumber must be dried to remove a relatively large percentage of water from the lumber. Acceptable water content will vary with the application as well as the type of wood, however, in many circumstances, a moisture content of 19% or less is an acceptable water content.
Although lumber may be air dried, kiln drying accelerates and provides increased control over the drying process. In kiln drying, charges of lumber are placed in a kiln chamber. A typical kiln chamber is a generally rectangular building which can be sealed to control the introduction and exhaust of air. Further, such kiln chambers typically have reversible fans for circulating the air through the chamber.
The charge of lumber placed in the kiln generally consists of a number of rectangular solid stacks of lumber. Each stack of lumber, in turn, typically consists of a number of vertically stacked, horizontal rows of lumber that form a rectangular solid. The horizontal rows are spaced apart for air to pass between the rows using wooden boards referred to as "stickers" that have a relatively small lateral cross-sectional area in relation to the lateral cross-sectional area of the lumber forming the charge. The stickers are generally spaced apart between each horizontal row to allow air to flow between the rows.
Typically, the stacks of lumber are placed on separate wheeled, flat bed cars which are mounted for movement on railroad-type tracks. Kilns may have any desired number of tracks. Multi-track kilns may therefore accept several stacks of lumber during each drying cycle.
In operation, a charge of green lumber is initially placed in a kiln chamber. After sealing the kiln chamber, the air within the kiln is heated to facilitate drying. The air may be heated in a number of ways such as by heat transfer from pipes extending through the kiln chamber in which steam flows. Alternatively, heated air may be introduced such as from a furnace. Kilns which utilize the introduction of heated air are typically referred to as direct fired kilns.
Fans generally positioned in upper portions of the kiln and above the stacked lumber circulate the heated air through the kiln chamber, including the stacks of lumber. Because the stickers provide spacing between the horizontal rows of lumber, the heated air passes between the rows of lumber and is in direct contact with both the upper and lower surfaces of the individual pieces of lumber. The fans continually recirculate the air through the kiln and the lumber to further dry the lumber. Periodically, a portion of the circulating air is exhausted from the kiln and additional air is introduced into the kiln. The additional air is typically heated in the kiln chamber, such as by heat transfer from the steam pipes. This periodic exhaust and replacement process allows circulating air which has absorbed a large amount of moisture from the green lumber to be removed, while drier air is introduced to accelerate the lumber's drying.
Within such kilns, the circulating air flows in a generally circular pattern. More particularly, fans above the stacks of lumber direct air laterally over the top of the lumber in a first direction. When the air contacts a first sidewall of the kiln chamber, the bulk of the air is forced downward by the fans and the ceiling of the kiln chamber. The circulating air subsequently flows through the spaces between the horizontal rows of lumber established by the stickers in a second, lateral direction opposite the first lateral direction. Upon contact with a second sidewall of the kiln chamber, the air rises and is recirculated by the fans through the lumber. Periodically, the fans are reversed such that the air flows in the opposite direction to provide generally consistent drying of the lumber.
In standard multi-track kilns, adjacent stacks of lumber typically define a narrow, vertically extending air space between the stacks. The circulating heated air will rise in this air space between adjacent first and second stacks of lumber. Depending upon the width of the air space, the air's circulation speed, and the distance between the circulating air and the top of the stacks when the air enters the air space from the first stack of lumber, the air may rise above the stacks of lumber. Consequently, such air would be recirculated without ever passing through the second stack of lumber.
As a result, the second stack of lumber would not be dried as thoroughly as the first stack of lumber. This uneven drying of the stacks of lumber may result in the production of inferior grades of lumber, including lumber with increased warping and cracking. If the drying is continued until the second stack of lumber is sufficiently dry in an attempt to avoid the production of inferior grades of lumber, the time and expense required to dry the lumber will significantly increase and the first stack of lumber may actually be dried too much. Further, as the same amount of air does not circulate through each stack of lumber, the consistency and predictability with which the lumber may be dried, i.e., the determination of appropriate drying times for which the lumber must remain in the kiln to ensure adequate moisture removal, is hindered.